In the year 2018, a near indestructible monster known as a Heterodyne attacked Japan. Impervious to the army’s missiles, it was finally decided that there was only way to stop it- by launching a nuclear OE warhead. Despite the awful cost in human lives, the weapon was deployed. Watching these events from afar, a little girl named Ibuki has no idea of the significance this event will have on her future.
It soon became clear that a weapon was needed to fight the Heterodyne, and a company named 21st Century Security was contracted to build a giant robot called Dai-Guard. But as the years passed, and the Heterodyne never returned, Dai-Guard was turned over to PR Division 2, where now it acts as little more than a tourist attraction and a sales gimmick.
The year is now 2030, and 21st Century Security is holding its security expo, showing off all its latest gadgets and gizmos. The star of the show, however, is Dai-Guard, and it is the job of PR Division workers Ibuki and Akagi (who gets to wear a bird suit) to show off Dai-Guard and hand out balloons. Not everyone is impressed by the giant robot, however, and when Akagi hears a group of children making of fun of both the robot and his bird suit, he decides to teach them a lesson. Dragging them over to a lower ventilation hatch, he activates the air flow, knocking them flat.
Getting his own back on the children cheers Akagi up a bit- at least until Ibuki reprimands him, and division chief Oosugi decides to dock his pay to cover the cost of blowing open the hatch. Worse yet, co-worker Aoyama has managed to land himself a sweet job handing out flyers to young woman; could the day get any worse?
Naturally, matters are about to get a whole lot worse. The Mount Fuji observatory has just decided a large undersea quake, and the implication is that it may be a prelude to another Heterodyne attack. Not everyone is willing to believe that, however; having grown complacent over the last twelve years, the last thing 21st Century Security’s board of directors wants to contemplate is another Heterodyne attack. Still, as per regulations, they have to remain on alert until the emergency passes.
In fact, the emergency is very real, and even as Akagi complains to Ibuki about how life is passing him by, they are distracted by a disturbance out to sea. As they turn to watch, their worst fears are confirmed as a monster emerges. Looking like a cross between a manta ray and a shark, with a red globe embedded in its face, there can be no doubt as to its identity- this is a new Heterodyne.
As the crowd begins running away, Akagi is insistent- they must use Dai-Guard to fight the Heterodyne. The rest of PR Division 2 isn’t so sure- even in the unlikely event that Dai-Guard could stand up to the monster, they don’t even have permission to take off. As far as the chief is concerned, it is better to leave things in the hands of the Anzen Hoshou army.
The board of directors agrees, which is why they have ordered Dai-Guard and PR Division 2 to evacuate immediately, but even as everyone else prepares to leave, Akagi is adamant. He is going to pilot Dai-Guard, with or without the board’s permission- although he won’t be able to do anything without Ibuki and Aoyama, the other two operators of the giant robot.
The army’s attack on the Heterodyne isn’t going well, and as Akagi watches, one of their helicopters crashes into the ground, setting fire to the nearby cars. Realising that there are people trapped down there, he becomes even more convinced that fighting back with Dai-Guard is the right thing to do. Recalling the destruction of twelve years ago, Ibuki agrees to join him, and even Aoyama cannot ignore the plight of the trapped crowd- although he only agrees to help as long as they restrict themselves to rescuing the trapped people and forgetting about any heterodyne-fighting heroics.
Climbing in, the three operators take their positions- Akagi as main pilot, Ibuki as navigator, and Aoyama as engineer- and activate Dai-Guard. Taking its first steps, the robot strides into the fire, where Aoyama suggests that they use a billboard to sweep away all the burning cars; Akagi complies, but no sooner have they finished making an escape route for the trapped crowd than they run smack into the approaching Heterodyne.
Reaching out with two whip-like tentacles, the Heterodyne knocks Dai-Guard back into a building, and even as Akagi straightens the robot up, it pushes them back once more. Despite Aoyama’s wish not to fight, it looks like they have no choice- especially as Dai-Guard and the Heterodyne are now blocking the very escape route they made. Realising this, Aoyama cranks Dai-Guard up to full power, and Akagi pushes the monster back, only to get dragged along with it when the Heterodyne grabs Dai-Guard with one of its tentacles.
Aware that they need to buy enough time for the crowd to escape, Ibuki tells Akagi to hold down the Heterodyne. As the crowd runs, Akagi repeatedly punches the Heterodyne, hoping to knock it out. His blows look impressive, but are ultimately ineffectual; the Heterodyne surges back up, and soon it and Dai-Guard are once again caught in a shoving match.
Dai-Guard isn’t holding up well against the strain, and with no hope of beating the Heterodyne, Chief Oosugi orders them to switch tactics and put all their effort into pushing the Heterodyne back into the sea. Using its full power, Dai-Guard successfully knocks the Heterodyne back, but even as it slides down the cliff, it grabs onto Dai-Guards right arm with one of its tentacles. A well-aimed punch with the left arm knocks the Heterodyne off balance, and it finally falls into the sea- unfortunately taking Dai-Guard’s right arm with it.
When the Heterodyne shows no sign of resurfacing, it looks like Dai-Guard might have just won its first battle. Still, aside from the chairman, neither the army nor the board of directors seem all that impressed. Is this victory going to turn out to be more trouble than it was worth?